Pre-defined variables are included within php and do not need to be manually defined like a standard variable.
Unlike standard variables however, pre-defined ones have one set value, typically correlating to server properties as demonstrated below.
Apache
The SERVER_SOFTWARE variable is the web application that is running PHP on the server, and when echoed will print that information.
129.89.43.13
The SERVER_ADDR variable returns the IP address of the server the script is being executed on currently and when echoed.
Pre-defined variables can be assigned to regular variables in order to make it more concise when calling it multiple times, while not necessary it does keep the code nicely organized.
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The pre-defined REQUEST_TIME variable was used above to get the timestamp of the last request, in the code however instead of being directly called, it was assigned to a variable named $time_req and echoed.